The invention relates to photodetectors and particularly to the field of avalanche photodetectors.
The noise factor of an avalanche photodetector, a measure of the noise degradation of the photodetector as compared to that of an ideal noiseless amplifier, increases considerably with the avalanche gain. In an avalanche photodiode the noise factor of the carrier multiplication process depends both on the ratio between the ionization coefficients (the ionization probability per unit length) for electrons and for holes and on the mechanism which initiates the carrier multiplication. A large difference between ionization coefficients for electrons and holes is beneficial for low noise provided the avalanche mechanism is initiated by the carrier type, electron or hole, having the higher ionization coefficient.
Although silicon exhibits a very large difference between the ionization coefficients of electrons and holes, (especially at low fields), the response of silicon devices to photons does not extend much beyond 1.1 microns; being basically limited by the 1.12 eV bandgap energy of the silicon.
Germanium avalanche photodiodes appear to be well suited for detection of photons in the wavelength range of 1.1-1.5 microns. However, germanium has almost equal electron and hole ionization coefficients which causes these devices to suffer from excess noise.
Avalanche photodiodes fabricated out of III-V semiconductor components, having radiation wavelength sensitivities which are adjustable in the region of practical interest, i.e., the low-loss spectral transmission window for optical fibers, also suffer from high noise due to the near equality of the ionization coefficients for holes and electrons.